Combined cycle power generation plant and cooling steam supply method thereof

ABSTRACT

A combined cycle power generation plant includes a gas turbine plant, a steam turbine plant operatively connected to the gas turbine plant, an exhaust gas heat recovery boiler for generating steam for driving the steam turbine plant by an exhaust gas of the gas turbine plant, an evaporator unit accommodated in the exhaust gas heat recovery boiler, the evaporator unit being divided into a first evaporator and a second evaporator, and a superheater provided for at least one of an intermediate portion between the first evaporator and the second evaporator and a portion on a downstream side of the second evaporator.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a combined cycle power generation plantcapable of setting a steam generated from an exhaust gas heat recoveryboiler to a proper temperature and supplying the steam to a steamturbine plant while supplying the steam generated from the exhaust gasheat recovery boiler to a gas turbine plant as a cooling steam, and alsorelates to a cooling steam supply method for the combined cycle powergeneration plant.

2. Description of the Related Art

In recent years, a study and development for obtaining high power andachieving high heat efficiency has been made in a combined cycle powergeneration plant. With the study and development, there has been made aplan to raise a combustion gas temperature of at least a portion of agas turbine inlet from a temperature of 1300° C., obtained in the priorart, to a temperature of 1500° C. or more.

In the case of creating a high temperature of the combustion gas of thegas turbine inlet, for example, a high chromium steel has beenconventionally used as a component of a gas turbine plant, and part ofthe compressed air from an air compressor has been supplied to thecomponent of the gas turbine plant as a cooling medium. However, in theprior art as described above, the strength of the component has beenclose to its limit. For this reason, in order to discover a coolingmedium substituting for the compressed air used in the prior art, it hasbeen attempted to study and develop a new cooling medium to be suppliedto the components of the gas turbine plant, and steam has been selectedas one of the cooling medium. A combined cycle power generation plantwhich takes advantage of steam cooling has been already disclosed in,for example, Japanese Laid-Open Patent Publication Nos. 5-163960 and6-93879.

Steam has a higher specific heat as compared with compressed air and isadapted to an absorption of heat generated in components, for example,in a gas turbine stationary blade and a movable blade, accompanying withhigh temperature of the gas turbine plant. However, each of the gasturbine stationary blade and the movable blade has a structure in whicha complicatedly meandering narrow passage is defined in the interior ofthese blades. For this reason, if impurities such as silica or the likeare contained in a steam passing through the above passage, unbalancedcooling occurs because of the possibility of clogging the passage withsilica or the like. As a result, these blades are broken down due tothermal strain accompanying the unbalanced cooling. Therefore, coolingsteam is required having a high cleanliness factor.

Further, in the case where a cooling steam is supplied to components ofthe gas turbine plant, it is necessary to provide a steam supply sourcewhich can supply a steam of proper temperature. If not so, the componentof the gas turbine plant generates an excessive thermal stress resultingfrom the difference in temperature between a combustion gas as a drivingfluid and these components, which difference may result in a possibilitythat these components are broken down. For this reason, in thecomponents of the gas turbine plant, a steam supply source, which cansupply a steam of proper temperature, is securely required.

On the other hand, with a temperature of the gas turbine plant beinghigh, a steam supplied from the exhaust gas heat recovery boiler to asteam turbine plant also has a high temperature. In this case, if thesteam temperature is too high, an excessive thermal stress is generatedin the steam turbine plant, and as a result, it becomes difficult tomaintain a material strength of the components of the steam turbineplant. For this reason, in the steam turbine plant, it is necessary toprovide a steam supply source which can supply a steam of a propertemperature.

As described above, in the combined cycle power generation plant, afirst high pressure superheater of the exhaust gas heat recovery boileris selected and set as a steam supply source, taking into considerationthe cleanliness of cooling steam, supply of proper temperature steam,and technical matters indispensable to the gas turbine and steam turbineplant. As one example, a combined cycle power generation plan as shownin FIG. 6 has been already proposed.

The combined cycle power generation plant shown in FIG. 6 has anarrangement in which a gas turbine plant 1 and a steam turbine plant 2are combined by a common rotary shaft 3 and an exhaust gas heat recoveryboiler 4 is located independently from these plants.

The gas turbine plant 1 includes a generator 5, an air compressor 6, acombustor 7 and a gas turbine 8. Air AR sucked by the air compressor 6is made into a high pressure compressed air, and is guided to thecombustor 7. In the combustor 7, a fuel is added to the compressed airso that a combustion gas is generated, and then, the combustion gas isexpanded by the gas turbine 8, thus the generator 5 is driven by thepower generated in the above manner.

The steam turbine plant 2 includes a high pressure turbine 9, anintermediate pressure turbine 10, a low pressure turbine 11 and acondenser 12. An exhaust steam, after being expanded by the highpressure turbine 9, is led to a reheater 13 of the exhaust gas heatrecovery boiler 4 and is superheated therein. Then, the exhaust steam isled to the intermediate pressure turbine 10 and is expanded as a reheatsteam. Further, the exhaust steam is again expanded by the low pressureturbine 11, and thereafter, is condensed into a condensate by thecondenser 12. The condensate is supplied as a feed water to the exhaustgas heat recovery boiler 4 via a pump 100.

Meanwhile, the exhaust gas heat recovery boiler 4 is provided with athird high pressure superheater 14, the reheater 13, a second highpressure superheater 15, a first high pressure superheater 16, a highpressure evaporator 18 including a high pressure drum 17, anintermediate pressure superheater 19, a high pressure economizer 20, alow pressure superheater 21, an intermediate pressure evaporator 23including an intermediate pressure drum 22, an intermediate pressureeconomizer 24, a low pressure evaporator 26 including a low pressuredrum 25, and a low pressure economizer 27. These components or elementsare arranged in order from an upstream side toward a downstream sidealong a flow of an exhaust gas G of the gas turbine plant 1, and steamis generated through the heat exchanging operation between each heatexchanger and the exhaust gas G.

Specifically, in the exhaust gas heat recovery boiler 4, a feed watersupplied from the condenser 12 of the steam turbine plant 2 via the pump100 is preheated by the low pressure economizer 27 and is led to the lowpressure drum 25. Then, by taking advantage of a difference in densityof drum water, the feed water is circulated through the low pressureevaporator 26 to generate steam, and the generated steam is supplied tothe low pressure turbine 11 via the low pressure superheater 21.

The low pressure economizer 27 leads part of the feed water, which isdiverted (divided) on an outlet side of the economizer 27, to the lowpressure drum 22 by a low pressure pump 28 and the intermediate pressureeconomizer 24. Due to a difference in density of drum water, a part ofthe saturated water is circulated through the low pressure evaporator 23to generate steam, and then, the generated steam is supplied to the gasturbine plant 1 via the intermediate pressure superheater 19 so as tocool the components of the gas turbine 8.

Further, the low pressure economizer 27 leads the remaining feed waterto the high pressure drum 17 by a high pressure pump 29 and the highpressure economizer 20. Then, the remaining saturated water iscirculated through the high pressure evaporator 18 to generate steam,and the generated steam is led to the first high pressure superheater16.

This first high pressure superheater 16 includes a steam pipe 30 forleading steam to the second high pressure superheater 15, and a bypasspipe 32 between which a bypass valve 31 is interposed. Steam passedthrough the bypass pipe 32 is joined together with a superheated steamgenerated by the second high pressure superheater 15, and after thetemperature of the steam has been decreased to a proper temperature, thesteam is supplied to the high pressure turbine 9 of the steam turbineplant 2 via the third high pressure superheater 14.

As described above, in the known combined cycle power generation plant,in the case where steam is supplied from the exhaust gas heat recoveryboiler 4 to the high pressure turbine 9, the first high pressuresuperheater 16 is set as the steam supply source. When the steamgenerated from the first high pressure superheater 16 is made into asuperheated steam by the second high pressure superheater 15, the steamtemperature is decreased by the bypass pipe 32, and then, thesuperheated steam having a proper temperature is supplied from the thirdhigh pressure superheater 14 to the high pressure turbine 9.

Moreover, when supplying a cooling steam to the components of the gasturbine 8, in the exhaust gas heat recovery boiler 4, a superheatedsteam generated by the intermediate pressure superheater 19 and anexhaust steam of the high pressure turbine 9 are joined together, andthen, the joined steam is supplied to the gas turbine 8 so that thestrength of the gas turbine members can be maintained so as to adapt tohigh temperature of a combustion gas on an inlet of the gas turbine 8.Further, a steam, which cooled the components of the gas turbine 8, isthen led to the intermediate pressure turbine 10 together with areheated steam of the reheater 13.

Meanwhile, in the combined cycle power generation plant shown in FIG. 6,during the start-up operation, the steam is still not generated from theexhaust gas heat recovery boiler, and for this reason, a cooling steamcannot be supplied to the gas turbine 8 from the intermediate pressuresuperheater 19 and the high pressure turbine 9. Thus, in order to coolthe components of the gas turbine 8, there is the following plan formaking use of the steam remaining in the high pressure drum 17 of theexhaust gas heat recovery boiler 4. Specifically, in this case, theexhaust gas heat recovery boiler 4 can make use of a residual heat ofthe first high pressure superheater 16, the second high pressuresuperheater 15 and the third high pressure superheater 14. Therefore, asshown in FIG. 7, an outlet side of the first high pressure superheater16 is provided with a cooling steam pipe 34 which is arranged parallelto the bypass pipe 32 and includes a control valve 33.

The residual steam of the high pressure drum 17 is led to the first highpressure superheater 16 so as to be superheated, and then, part of theresidual steam is guided to the second high pressure superheater 15 andthe first high pressure superheater 14 while the remaining steam thereofis led to the cooling steam pipe 34. Subsequently, the two flows ofsteams are joined together on the outlet side of the third high pressuresuperheater 14, and a high-temperature portion of the gas turbine 8 istemporarily cooled by the joined steam. When the gas turbine plant 1 isin a high-load state, the components of the gas turbine 8 are cooled bythe joined steam of the intermediate pressure superheater 19 and thehigh pressure turbine 9.

As described above, in the combined cycle power generation plant shownin FIG. 6, the known plan mentioned above has been performed such that asteam of a proper temperature is supplied from the exhaust gas heatrecovery boiler 4 to the high pressure turbine 9 during the ratedoperation. However, when the gas turbine plant 1 is in a state of apartial load operation, the exhaust gas G supplied from the gas turbine8 to the exhaust gas heat recovery boiler 4 is further increased in itstemperature.

In general, in the case where the partial load operation of the gasturbine plant 1 is carried out, as shown by a broken line in FIG. 8, atemperature of the exhaust gas G rises. In contrast to the rising of thetemperature of the exhaust gas G, the steam temperature of the firsthigh pressure superheater 16 is substantially constant as shown by adotted chain line in FIG. 8. On the other hand, the steam temperature ofthe second high pressure superheater 15 becomes high as shown by a solidline in FIG. 8. The steam temperature of the third high pressuresuperheater 14 rises, not shown, like the second high pressuresuperheater 15. In this case, the exhaust gas heat recovery boiler 4sets the superheated steam of the third high pressure super heater 14 ata proper temperature and supplies it to the high pressure turbine 9.Thus, when supplying the superheated steam of the first high pressuresuperheater 16 to the third high pressure superheater 14 via the bypasspipe 32, a bypass steam flow rate is increased as shown by a solid linein FIG. 8. For this reason, a heat exchange quantity of the second highpressure superheater 15 is increased as the exhaust gas reaches hightemperature. However, a steam quantity of any heated object remarkablydecreases, and during the heat exchange, an excessive thermal stress isgenerated due to a biased temperature distribution. As a result, aproblem is caused such that a heat transfer pipe is burned or brokendown.

On the other hand, the combined cycle power generation plant shown inFIG. 7 is constructed as follows. Specifically, during a start-upoperation, the outlet side of the first high pressure superheater 16 isprovided with the cooling steam pipe 34 which is arranged parallel tothe bypass pipe 32. Due to the steam remaining in the high pressure drum17, the steam is led to the first high pressure superheater 15, which isused as a cooling steam supply source. Further, part of the steam issupplied to the cooling steam pipe 34 while the remainder thereof issupplied to the third high pressure superheater 14 via the second highpressure superheater 15. Subsequently, both the steam flows are joinedtogether on the outlet side of the third high pressure superheater 14,and the joined steam is supplied to the gas turbine 8 so as to cool thecomponents of the gas turbine 8.

However, even during the start-up operation, for example, when the gasturbine plant 1 is in a hot start or a very hot start state, a residualheat of each heat exchanger is still at a high temperature, and for thisreason, there sometimes arises a case where the temperature of thecooling steam exceeds a proper cooling steam temperature of the gasturbine 8. In order to realize a proper temperature of the coolingsteam, as shown in FIG. 7, it is necessary to locate a steam generatingapparatus 35, which generates a steam having a relatively lowtemperature, on an inlet side of the gas turbine 8. However, thisarrangement is not advantageous when the cost of facilities isconsidered.

In FIG. 7, the outlet side of the gas turbine 8 is provided with a firstbypass pipe 12 a connected to the condenser 12. Further, the inlet sideof the high pressure turbine 9 is provided with a second bypass pipe 12b connected to the condenser 12.

As described above, in the known combined cycle power generation plantsshown in FIGS. 6 and 7, the following plan has been made. Specifically,the first high pressure superheater 16, which generates steam having astable temperature with respect to a load variation as shown in FIG. 8,is set as a steam supply source, and steam having a proper temperatureis supplied to the high pressure turbine 9 therefrom while a coolingsteam having a proper temperature is supplied to the gas turbine 8.Considering the details, however, the conventional combined cycle powergeneration plants have various problems as described above, and it isrequired to achieve improvements for sufficiently coping with the hightemperature of the gas turbine plant.

SUMMARY OF THE INVENTION

A primary object of the present invention is to substantially eliminatedefects or drawbacks encountered in the prior art mentioned above and toprovide a combined cycle power generation plant which can stably supplysteam having a proper temperature from an exhaust gas heat recoveryboiler to a steam turbine plant even during a partial load operation ofthe turbine plant.

Another object of the present invention is to provided a combined cyclepower generation plant which can supply a cooling steam having a propertemperature from an exhaust gas heat recovery boiler to a gas turbineplant even during a start-up operation.

A further object of the present invention is to provide a cooling steamsupply method of a combined cycle power generation plant capable ofsupplying a cooling steam generated from an exhaust gas heat recoveryboiler to another plant.

These and other objects can be achieved according to the presentinvention by providing, in one aspect, a combined cycle power generationplant comprising:

a gas turbine plant;

a steam turbine plant operatively connected to the gas turbine plant;

an exhaust gas heat recovery boiler for generating steam for driving thesteam turbine plant by an exhaust gas of the gas turbine plant;

an evaporator unit accommodated in the exhaust gas heat recovery boiler,said evaporator unit being divided into a first evaporator disposed onan upstream side of an exhaust gas flow and a second evaporator disposedon a downstream side thereof, and

a superheater unit provided at at least one of an intermediate positionbetween the first evaporator and the second evaporator and a position ona downstream side of the exhaust gas flow of the second evaporator.

In preferred embodiments, the evaporator unit includes at least one lowpressure evaporator and one high pressure evaporator of a pressurehigher than that of the lower pressure evaporator and the superheaterunit includes at least one low pressure superheater and one highpressure superheater of a pressure higher than that of the lowerpressure superheater. The superheater unit is located adjacent to aportion, at which a superheated steam having a low degree of superheatis generated, at an intermediate portion between the first and secondevaporators.

There is further provided with another superheater unit disposed on anupstream side of the first mentioned superheater unit and a bypass pipeconnected to the first mentioned superheater unit and adapted to join asuperheated steam generated from the another superheater unit to asuperheated steam generated from the first mentioned superheater unit.The bypass pipe is provided with a bypass valve.

The superheater unit makes use of a superheated steam generatedtherefrom as a process steam for another plant. The superheater unitutilizes the generated superheated steam as a cooling steam for a gasturbine constituting a gas turbine plant.

The combined cycle power generation plant further comprises a steamsupply means for supplying a steam composed of a steam from the exhaustgas heat recovery boiler and a steam from the steam turbine to the gasturbine plant as a cooling steam and a control means for controlling thesteam supply of the steam supply means to the gas turbine plant.

In another aspect of the present invention, there is provided a coolingsteam supply method of a combined cycle power generation plant,comprising a gas turbine plant, a steam turbine plant, a superheater, anevaporator unit and an exhaust gas heat recovery boiler having a drum,which are operatively connected together, in which a cooling steam issupplied from the drum of the exhaust gas heat recovery boiler to thegas turbine plant, said method comprising the steps of:

supplying a steam remaining in the exhaust gas heat recovery boiler tothe gas turbine plant during a start-up operation thereof;

joining an exhaust steam generated from the steam turbine plant and asteam generated from the exhaust gas heat recovery boiler together afteropening a steam turbine inlet valve and leading the steam to the steamturbine; and

supplying the joined steam to the gas turbine plant as a cooling steam.

In performing the above method, in preferred embodiments, the evaporatorunit is divided into two evaporators and the drum is a high pressuredrum. Steam remaining in the high pressure drum is supplied to thesuperheater disposed at an intermediate portion between the divided twoevaporators and a steam generated from the superheater is supplied tothe gas turbine plant as a cooling steam. The superheater joins thesteam generated therefrom and a steam generated from another superheaterlocated on an upstream side of the first mentioned superheater whensupplying the generated steam to the gas turbine plant as a coolingsteam. The generated steam is supplied through a bypass pipe connectedto the first mentioned superheater.

According to the characteristic features and structures of the presentinvention mentioned above, the high pressure evaporator is divided intotwo high pressure evaporators. and the first high pressure superheateris provided on a position where a superheated vapor having a relativelylow degree of superheat is generated in the middle portion between thesehigh pressure evaporators. Further, the superheated steam generated fromthe first high pressure superheater is controlled, and thereafter, issupplied to the high pressure turbine. Therefore, even during a partialload operation, the superheated steam can be supplied to the highpressure turbine as a driving steam having a proper temperature withoutspecially providing temperature reducing means.

Further, in the cooling steam supply method of the combined cycle powergeneration plant according to the present invention, by making use ofthe steam of the high pressure drum, a superheated steam having arelatively low degree of superheat is generated by the first highpressure superheater, and then, the superheated steam is controlled andis supplied to the gas turbine. Therefore, even if during a start-upoperation, steam is not still generated from each heat exchanger of theexhaust gas heat recovery boiler, a cooling steam having a propertemperature can be securely supplied to the components of the gasturbine.

Furthermore, in the combined cycle power generation plant according tothe present invention, the superheated steam generated from the highpressure superheater is supplied to other plants as a process steam, sothat an effective heat use can be achieved.

Still furthermore, according to the present invention, the superheaterutilizes the generated superheated steam as a cooling steam for a gasturbine constituting the gas turbine plant.

The nature and further characteristic features of the present inventionwill be made clear from the following descriptions made with referenceto the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic system diagram showing a combined cycle powergeneration plant according to a first embodiment of the presentinvention;

FIG. 2 is a view for explaining a position where a first high pressuresuperheater according to the present invention is located;

FIG. 3 is a graph showing a steam temperature and a bypass steam flowrate distribution with respect to a load variation obtainable from thepresent invention;

FIG. 4 is a schematic system diagram for explaining a cooling steamsupply method of the combined cycle power generation plant according tothe present invention;

FIG. 5 is a schematic system diagram showing a combined cycle powergeneration plant according to a second embodiment of the presentinvention;

FIG. 6 is a schematic system diagram showing a combined cycle powergeneration plant of the prior art;

FIG. 7 is a schematic system diagram showing another combined cyclepower generation plant in the prior art; and

FIG. 8 is a graph showing a steam temperature and a bypass steam flowrate distribution with respect to a load variation in the prior art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A first embodiment of the present invention will be described hereunderwith reference to FIGS. 1 to 3.

Referring to FIG. 1, the combined cycle power generation plant accordingto the first embodiment has a construction in which a steam turbineplant 37 is combined with a gas turbine plant 36 by a drive shaft 38 sothat a cooling steam is supplied to the gas turbine plant 36, and anexhaust gas heat recovery boiler 39 which supplies a driving steam tothe steam turbine plant 37 is provided independently from these plants.

The gas turbine plant 36 includes a generator 40, an air compressor 41,a combustor 42, and a gas turbine 43. Air AR sucked by the compressor 41is made into a high pressure compressed air and is guided to thecombustor 42. In the combustor 42, a fuel is added to the compressed airso that a combustion gas is generated, and then, the combustion gas isexpanded by the gas turbine 43, thus the generator 40 is driven by thepower generated in the above manner.

The steam turbine plant 37 includes a high pressure turbine 44, anintermediate pressure turbine 45, a low pressure turbine 46 and acondenser 47. An exhaust steam after being expanded by the high pressureturbine 44 is guided to a reheater 48 of the exhaust gas heat recoveryboiler 39 and is heated therein. Then, the exhaust steam is led to theintermediate pressure turbine 45 and is expanded as reheated steam.Further, the exhaust steam is again expanded by the low pressure turbine46, and thereafter, is condensed into a condensate by the condenser 47.The condensate is supplied as a feed water to the exhaust gas heatrecovery boiler 39 via a pump 49.

Meanwhile, the exhaust gas heat recovery boiler 39 is provided with athird high pressure superheater 50, a reheater 48, a second highpressure superheater 51, a high pressure evaporator 53 including a highpressure drum 52, an intermediate pressure superheater 57, a highpressure economizer 58, a low pressure superheater 59, an intermediatepressure evaporator 61 including an intermediate pressure drum 60, anintermediate pressure economizer 62, a low pressure evaporator 64including a low pressure drum 63, and a low pressure economizer 65.These component elements are arranged in order from an upstream sidetoward a downstream side along a flow of an exhaust gas G of the gasturbine plant 43. Steam is generated through the heat exchangingoperation between each heat exchanger and the exhaust gas G. Further, alow pressure pump 68 supplies part of a saturated water of the lowpressure economizer 65 to the intermediate pressure economizer 62, and ahigh pressure pump 69 supplies a remaining part of the saturated waterof the low pressure economizer 65 to the high pressure economizer 58.

The high pressure evaporator 53 includes two divisional evaporators,that is, a second high pressure evaporator 54 and a first high pressureevaporator 55, and is further provided with a first high pressuresuperheater 56 which is arranged on a middle portion between the secondhigh pressure evaporator 54 and the first high pressure evaporator 55.The first high pressure superheater 56 is provided on a middle portionbetween the second high pressure evaporator 54 and the first highpressure evaporator 55. The reason for taking the above arrangement isas follows.

As shown in FIG. 2, it has been known that the exhaust gas G passingthrough the high pressure evaporator 53 has a saturation temperature of+170° C. on an inlet side of the high pressure evaporator 53, and thedifference in saturation temperature becomes small on the downstreamside from the central portion thereof. For this reason, in this firstembodiment, if the first high pressure superheater 56 is provided in aregion A where the exhaust gas has a saturation temperature of +30° C.to 10° C., it is possible to limit a degree of superheat of superheatedsteam generated from the first high pressure superheater 56 within arelatively low temperature range from 10°C. to 20° C. In this case, apinch point of the superheated steam and exhaust gas G generated fromthe first high pressure superheater 56 is about 8° C. according to atest calculation. Further, in the case where the steam turbine plant 37requires a superheated steam of a low degree of superheat such as 5° C.to 10°C., it is preferable that the first high pressure superheater 56is provided on the downstream side of the first high pressure evaporator55 which is one of the two divided evaporators of the high pressureevaporator 53.

An operation of the combined cycle power generation plant according tothe first embodiment of the present invention shown in FIG. 1 will bedescribed hereunder.

When the combined cycle power generation plant is in a partial loadoperation state, the temperature of the exhaust gas G supplied from thegas turbine plant 36 to the exhaust gas heat recovery boiler 39 rises,and with the rise of temperature, the temperature of steam generatedfrom each of the third high pressure superheater 50, the second highpressure superheater 51 and the first high pressure superheater 56 alsorises and exceeds a temperature of a driving steam required for the highpressure turbine 44 of the steam turbine plant 37.

However, in the present embodiment, the high pressure evaporator 53 isdivided into the second high pressure evaporator 54 and the first highpressure evaporator 55, and further, the first high pressure superheater56 is provided at the intermediate portion between the two divided highpressure evaporators 54 and 55. In this manner, the degree of superheatof the superheated steam generated from the first high pressuresuperheater 56 is limited within a range from 10° C. to 20° C. as shownin FIG. 2. In the case where part of the superheated steam generated inFIG. 1 from the first high pressure superheater 56 is supplied to abypass pipe 67 including a bypass valve 66 as bypass steam, the flowrate of the steam can be reduced, and also, the remaining superheatedsteam is supplied to the second high pressure superheater 51 so as tomake low a temperature of the superheated steam generated from thesecond high pressure superheater 51. According to a test calculation,the temperature of the superheated steam generated from the first highpressure superheater 56 of the present embodiment is about 100° C.,which is lower than the temperature (400° C. or more) of superheatedsteam generated from the known first high pressure superheater 16 shownin FIG. 6.

FIG. 3 is a characteristic chart showing a temperature distribution ofthe exhaust gas G shown by a broken line, a temperature distribution ofthe superheated vapor generated from the second high pressuresuperheater 51 shown by a solid line, a temperature distribution of thesuperheated steam generated from the first high pressure superheater 56shown by a dotted chain line, and a bypass steam flow rate shown by asolid line, with respect to a load variation of the gas turbine plant36.

As seen from FIG. 3, the temperature of the exhaust gas G rises withrespect to a load variation of the gas turbine plant 1. However, atemperature of the superheated steam generated from the second highpressure superheater 51 is lowered as compared with the rise of theexhaust gas temperature, and also, a bypass steam flow rate is reduced.

As described above in regard to FIG. 1, the first high pressuresuperheater 56 generates a superheated steam having a relatively lowdegree of superheat. Much of the superheated steam is supplied to thesecond high pressure superheater 51, and then, is supplied to the bypasspipe 67 so that the bypass steam flow rate becomes relatively low.Further, the bypass steam flow is joined together with the superheatedsteam generated from the second high pressure superheater 51 to lowerthe temperature of the steam. Subsequently, the low-temperature joinedsteam is changed into a proper temperature driving steam necessary for apartial load operation by the third high pressure superheater 50, andthen, is supplied to the high pressure turbine 44, and thus, the highpressure turbine 44 is driven. The high pressure turbine 44 expands thedriving steam so as to drive the generator 40 while joining the exhauststeam with steam generated from the intermediate pressure superheater57, and then, supplies the joined steam to the gas turbine 43. Thejoined steam cools the components of the gas turbine 43, and thereafter,is joined with a reheated steam generated from the reheater 48. Further,the joined steam is expanded by the intermediate pressure turbine 45,and thereafter, is supplied to the low pressure turbine 46.

In the present embodiment, the high pressure evaporator 53 is dividedinto two, that is, the second high pressure evaporator 54 and the firsthigh pressure evaporator 55, and the first high pressure superheater 56is located at an intermediate portion between these high pressureevaporators 54 and 55. Further, the degree of superheat of thesuperheated steam generated from the first high pressure superheater 56is made lower, whereby much of the superheated steam is supplied to thesecond high pressure superheater 51, so that a relatively littlesuperheated steam can be supplied to the bypass pipe 67 and that adriving steam of a proper temperature can be supplied to the highpressure turbine 44 without providing any temperature reducing means inthe third high pressure superheater 50.

At this time, since much of the superheated steam flows through thesecond high pressure superheater 51, it is possible to provide a uniformtemperature distribution such that a thermal stress is almost notgenerated during heat exchange. Therefore, the material strength of theheat transfer pipe can be maintained for the long term. Further, sincelittle of the superheated steam flows through the bypass pipe 67 ascompared with the conventional one, when selecting the bypass valve 66,it is possible to select a valve having a relatively smaller diameter,contributing to a reduction of cost.

In general, since the first high pressure superheater 56 is constructedso that impurities such as silica contained in the saturated steamsupplied from the high pressure drum 52 adhere to the heat transfer pipethereof, the first high pressure superheater 56 has a function ofimproving the cleanliness of the saturated steam. Thus, when supplying acooling steam to the components of the gas turbine 43, a cooling steamhaving a relatively high cleanliness can be supplied thereto, so thatthe components of the gas turbine 43 can be prevented from being cloggedwith silica or the like.

FIG. 4 is a schematic system diagram showing a cooling steam supplymethod of a combined cycle power generation plant according to anotherembodiment of the present invention. For simplification of explanation,like reference numerals are used to designate the same portions as thecomponents of the combined cycle power generation plant shown in FIG. 1.

Prior to an explanation about a cooling steam supply method of acombined cycle power generation plant according to this embodiment,first, the construction will be described below.

The high pressure evaporator 53 is divided into two, that is, the secondhigh pressure evaporator 54 and the first high pressure evaporator 55,and the first high pressure superheater 56 is provided at theintermediate portion between these high pressure evaporators 54 and 55.An outlet side of the first high pressure superheater 56 is providedwith a cooling steam pipe 71 between which a control valve 70 connectedto an inlet side of the gas turbine 43 is interposed. Further, theoutlet side of the gas turbine 43 is provided with a first bypass pipe72 connected to the condenser 47, and on the other hand, the inlet sideof the high pressure turbine 44 is provided with a second bypass pipe 73connected to the condenser 47.

In the combined cycle power generation plant, a daily start/stop (DSS)operation is frequently carried out unlike a conventional powergeneration plant. In the case of the DSS operation, the third highpressure superheater 50, the second high pressure superheater 51, thesecond high pressure evaporator 54 and the first high pressureevaporator 55 which divides the high pressure evaporator 53 into two,are accommodated in the exhaust gas heat recovery boiler 39, and theseelements are respectively kept at a warming state before the start-upoperation of the exhaust gas heat recovery boiler 39. Further, theresidual heat of these elements is at high temperature.

The high pressure drum 52, the intermediate drum 60 and the low pressuredrum 63 have a steam pressure of about 60 kg/cm², 12 kg/cm² and 4kg/cm², respectively.

In the case where a cooling steam is supplied to the components of thegas turbine 43, according to the test calculation taking various lossesinto consideration, the cooling steam is required to have a steampressure of 20 kg/cm². Further, the first high pressure superheater 56can reduce a degree of superheat of a steam supplied from the highpressure drum 52 from 10° C. to 20° C. Thus, since the first highpressure superheater 56 can set a temperature of the cooling steam toabout 300° C. during the hot start-up operation, the cooling steam meetsthe condition of 350° C. or less cooling steam temperature required forthe components of the gas turbine 43. Therefore, the cooling steam canbe sufficiently supplied to the gas turbine 43.

The construction of this embodiment is based on the test calculation asdescribed above, and a cooling steam supply method will be describedhereunder.

Before the start-up operation of the combined cycle power generationplant, first, the first high pressure superheater 56 supplies the steamof the high pressure drum 52 to the condenser 47 through the secondbypass pipe 73 by the second high pressure superheater 51 and the thirdhigh pressure superheater 50 so as to blow out impurities such as silicacontained in the respective high pressure superheaters 51 and 50. Next,the bypass valve 66 of the bypass pipe 67 is throttled to open thecontrol valve 70 of the cooling steam pipe 71, and then, the steam ofthe high pressure drum 52 is supplied to the gas turbine 43 as a coolingsteam. At this time, in the case where a thermal stress is generated inthe components of the gas turbine 43 because the cooling steamtemperature is too low, the control valve 74 provided on the outlet sideof the third high pressure superheater 50 is opened so that the coolingsteam is joined together with the steam of the third high pressuresuperheater 50, and thus, the joined steam temperature is controlled sothat the cooling steam becomes a proper temperature. The cooling steamcools the components of the gas turbine 43, and thereafter, is suppliedfrom the first bypass pipe 72 to the condenser 47, or is discharged froma chimney (not shown) of the exhaust gas heat recovery boiler 39.

When the start-up operation of the gas turbine 43 has been completed andthe steam turbine inlet valve is opened, the exhaust steam of the highpressure turbine 44 and the superheated steam of the intermediatepressure superheater 57 are joined together, and then, an operation forcooling the components of the gas turbine 43 is carried out with the useof the joined steam.

According to this method, the steam remaining in the high pressure drum52 is used, and when the steam is supplied from the first high pressuresuperheater 56 to the components of the gas turbine 43 as the coolingsteam, the cooling steam is controlled so as to become a propertemperature. Thus, even if each heat exchanger of the exhaust gas heatrecovery boiler 39 still does not generate a steam, it is possible tocool securely the components of the gas turbine 43, whereby a stableoperation of the gas turbine 43 can be carried out.

FIG. 5 is a schematic system diagram showing a combined cycle powergeneration plant according to a second embodiment of the presentinvention. In this second embodiment, like reference numerals are usedto designate the same portions as the components used in the firstembodiment shown in FIG. 1.

In the second embodiment, the high pressure evaporator 53 is dividedinto two, that is, the second high pressure evaporator 54 and the firsthigh pressure evaporator 55, and the first high pressure superheater 56located at the intermediate portion between these evaporators 54 and 55is provided with a process steam supply pipe 75. Thus, it is possible tosupply the steam of the first high pressure superheater 56 to agasification furnace such as a coal gasification, residual oilgasification plant or the like.

In the combined cycle power generation plant which does not supply acooling steam from the exhaust gas heat recovery boiler 39 to thecomponents of the gas turbine 43, in the case of making the steam of thethird high pressure superheater 50 into a proper temperature drivingsteam and supplying it to the high pressure turbine 44, the outlet sideof the second high pressure superheater 51 is provided with atemperature reducer 76. Then, the steam from the high pressureeconomizer 58 is supplied from a high-pressure superheatertemperature-reducing water pipe 77 to the temperature reducer 76 througha valve 78 which is controlled depending upon the outlet temperature ofthe second or third high pressure superheater 51 or 50. Further, thesuperheated vapor temperature of the second high pressure superheater 51is decreased, and the steam is supplied to the third high pressuresuperheater 50. For this reason, the superheated steam generated fromthe first high pressure superheater 56 has a steam pressure of 100kg/cm² and a temperature of about 350° C.

In this embodiment, the superheated steam generated from the first highpressure superheater 56 is supplied to a gasification furnace such as acoal gasification plant as process steam, so that the superheated steamgenerated from the exhaust gas heat recovery boiler can be effectivelyused. In particular, a gasification power generation plant recentlyreceives much attention, and it is very advantageous in a thermalefficiency calculation if the plant would received a superheated steamgenerated from the exhaust gas heat recovery boiler.

It is to be noted that the present invention is not limited to thedescribed embodiments and many other changes and modifications may bemade without departing from the scope and spirit of the appended claims.

What is claimed is:
 1. A combined cycle power generation plantcomprising: a gas turbine plant; a steam turbine plant operativelyconnected to the gas turbine plant; an exhaust gas heat recovery boilerconfigured to generate steam for driving the steam turbine plant by anexhaust gas of the gas turbine plant; a high pressure evaporator unitaccommodated in the exhaust gas heat recovery boiler, said high pressureevaporator unit being divided into a first high pressure evaporatordisposed on an upstream side with respect to an exhaust gas flow and asecond high pressure evaporator disposed on a downstream side thereof; ahigh pressure superheater unit including first, second and third highpressure superheaters disposed in series in this order from downstreamto upstream with respect to the exhaust gas flow, said first highpressure superheater being disposed at at least one of an intermediateposition between the first high pressure evaporator and the second highpressure evaporator and a position on the downstream side of the firsthigh pressure evaporator with respect to the exhaust gas flow; and abypass pipe configured to bypass the second high pressure superheaterand to connect the first high pressure superheater with the third highpressure superheater.
 2. A combined cycle power generation plantaccording to claim 1, further comprising: one low pressure evaporatorand one intermediate pressure evaporator disposed in the exhaust gasheat recovery boiler.
 3. A combined cycle power generation plantaccording to claim 1, wherein said bypass pipe is provided with a bypassvalve.
 4. A combined cycle power generation plant according to claim 1,wherein said high pressure superheater unit uses a superheated steamgenerated therefrom as a process steam for another plant.
 5. A combinedcycle power generation plant according to claim 1, wherein said highpressure superheater unit utilizes generated superheated steam as acooling steam for a gas turbine constituting the gas turbine plant.
 6. Acombined cycle power generation plant according to claim 1, furthercomprising: a pipe configured to supply a steam composed of a steam fromthe exhaust gas heat recovery boiler and a steam from the steam turbineto the gas turbine plant as a cooling steam; and a device configured tocontrol steam supply of the pipe to the gas turbine plant.